Reputable dealers will not only check people out before selling them, but will also visit at home to check on the area it will be used, storage, charging points etc.
Remember the days before bikes were limited? A quick twenty minutes with whatever mechanic didn't happen to be busy and you could take 1,000cc onto the North Circular (and many did!).
We have bought several scooters from local dealers, and even though they know we use scooters they still insist on checking us out.
The troble is there are too many get rich quick merchants out to make a quick buck from a vulnerable person.
That's why you will often see people struggling with the small three wheel scooters in normal street work when they are really just for indoors and shopping centres and the like.
Difficult to understand the loss of control if it was a normal straight road - they all operate the "dead man's handle" system, as soon as you let go of, or relax, the control it stops dead.
Perhaps she simply froze on the control, this happens with cars, bikes, even aeroplanes. No way of controlling that of course.
The "banning" option is fraught with difficulties. If you do it you have to have alternative arrangements in place or you could be on the wrong end of discrimination proceedings.
My wife and I both know there are some places to which we simply can't go, but others take the view that if wheelchairs can get to the top of Snowdon or Cairngorm then they should be able to shop wherever they want.